Method for purifying tinny leads and lead-tin alloys



Paggpted Apr. 1, 1941 METHOD FOR PURIFYING TINNY AND LEAD-TIN ALLOYS r OFFICE LEADS Gustave E. Behr, New York, N. Y., assignor to National Lead Company, New York,

N. Y., a

corporation of New Jersey No Drawing. Application July 26, 1939, V Serial No. 286,565

' 4 Claims. (01. 75-78) My present invention relatesto a method for purifying metals. Its object is the purification of such metals as tinny leadsand alloys of leadtin, particularly solder stock.

The present application is a continuation-in part of my 'copending application, Serial No.

218,952, filed July 13, 1938, now Patent 2,196,050,

granted April 2, 1940. I g e 1 According to refining practices commonly in vogue, various metal reagents, such as calcium,

magnesium, sodium and aluminum are used as reagentsin'the treatment of impure lead-tin alloys and tinny leads to remove one or more basic impurities. In such instances, a small portion of one or more of the above mentioned refining agents are retained within the partially purified metal, and must .be removed in order to meet the requirements of the trade for lead-tin alloys. Zinc also is often found in such lead-tin-alloys and must'likewise be removed. Therefore, in

order to, render such contaminated alloys suit:

able for-commercial-adaptation, it is generally necessary to remove the contaminants present without substantially aflecting the principal'alloy components, namely lead and tin. This procedure has often proved troublesome and my present invention constitutes a simple, economical and eflicient method for removing these impurities. e

I have found that by treating molten tinny leads or alloys of lead tin, as hereinafter de-' use of lead chloride ina process for removing tin, whereas my present invention is directed toward the purification of tinny leads and lead-tin alloys without substantially affecting the lead-tin content of the purified metal. It is to be further noted that my novel process does not employ any, oxidizing agent as shown in U. S. Patents Nos. 2,043,573 and 2,115,299.

While my invention in its broadest aspects, as hereinabove set forth,, embraces the use of any lead halide, such as lead fluoride, lead bromide, etc., or.a mixture of two or more of such compounds, for the sake of simplicity my invention will be particularly described with respect to the use of lead chloride. It will, of course, be understood by those-skilled in this art that the conditions such as temperature at which the halide treatment is" carried out may have to be. varied, i. e., raised or lowered somewhat, depending upon whether the lead halide has a melting point above or below that of lead chloride.

removed from lead-tin alloys and tinny leads by means of a simple. treatment, according to my scribed, with a lead halide, for example, lead chloride (PbClzl in the presence of .a mixture of two or more melting point depressing'salts, the

metal will be purifiedat relatively low temperature of all elements having greater aflinity for the halogen than the lead; that is to say, all ele-' ments which are electropositive to lead with respect to the halogen under the conditions of the melting pot. My discovery is a general one; thus,

all elements to .be removed are converted in the process of my inventioninto the respective halide tin alloys because the'addition of lead does not present invention, are the following: The alkali and alkaline earth metals, magnesium, beryllium, aluminum-and zinc. It may be noted'in this connection that the elements in the list just mentioned when present in lead-tin alloys, are oxi dizable as, for example, by blowing through the molten alloy a current of air or steamand can be in this way removed from the alloy in the form 'of an oxidic dross. Such a procedure, however,

would 'not be desirable in. purifying tinny leads,

F inasmuch as it would involve losing considerable of the tin content as an oxidic dross. My present invention, therefore, constitutes an alternative method for the removal of these elements from tinny leads and lead-tin alloys.

My invention may'very easily be carried out in any suitable type of apparatus; the ordinary lead melting pot, equipped with means for agitation,

and lead as metal is added to the alloy. It is at is most convenient; In carrying out the invention, the tinny lead tobe purified is placed in the reaction vessel and heated to above its melting point. 1 Lead-chloride together withthe meltconstitute a contamination of the alloy being treated. Y

I am aware that the prior arthas taughtmany ways of removing, impurities from metals. However, none ofthese show my novelprocess. For

- example, U. S. Patents Nos. 2,113,643 and'2,091,-

981 show the removalof impuritiesby a chloride 'ing point depressing salts is then added to'the molten 'metal while agitating the molten mass. Reaction at once takes place betweenthe lead chloride and the element to be removed, resulting in the formation of a chloride slag of the element removed. The temperature at which the process is to be carried out should preferably be above the melting point of the mixture of lead chloride andmelting point depressing salts. After The halides of the alkali metals, for example,

potassium and sodium chloride, are particularly eifective. For instance, the mixture of a sumcient amount of sodium chloride to form the eutectic mixture of about 92 percent lead chloride and .-8 percent sodium chloride will give amelting point of 410 C. instead of about 501 C. for pure lead chloride; also, the ternary mixture of about 91.2 percent lead chloride, 3.8 percent potassium chloride and 4.9 percent-sodium chloride will give a melting point of 383 C; instead of about 501 C. for pure lead chloride. In selecting the salt for lowering the melting point of lead chloride in'the practice of my invention, it is necessary that the salt selected be nonreactive both withrespect to the metal to be purified, as well as to the lead chloride. These salts I have elected to designate as non-reactive melting point depressingsaltsf, These will generally be used in lesser amounts than the lead chloride.

The amount of lead chloride to be employed will depend upon the amount of impurity to be removed. Although the purification of the metal will go to substantial completion by the use of a stoichiometric quantity, it might be preferable to employ-an excess over this quantity in order-to insure the complete removal of impurities. n the other hand, if for some reason it is not desired or necessary to eflect the complete removal of the impurities present in the lead-tin alloys treated; less than the stoichiometric quantity of lead chloride may be employed. It is also evident that the removal of the impurities may be accomplished in more than one step by successive treatments, if desired.

The removal of the impurities is facilitated by stirring the lead chloride-meltingpoint depressing salt mixtures and the molten alloy; the time of stirring may vary according to the quantity of' the tinny leads or lead-tin alloy treated and the impurity to be removed. The time required for the reaction cannot be definitely given for every case, but those skilled in this art will be enabled from the description given here to carry out my invention to effect a substantially complete removal of these impurities, 7 As an illustrative example of the application of my process I ive the following:

Example I:

About 1000 grams of a solder metal consisting essentially of about 50 percent lead and 50 percent tin but containing'0.41 grams of, aluminum were'placed in an ordinary laboratory lead melting pot and heated to a red heat. The calculated quantity ofv lead chloride required to combine 'with and remove 0.4 grams of aluminum according to the equation:

would be about 6.2 grams. However, in this experiment, when the lead had reached a red heat,

711 grams of a solder metal consisting essen-. tially of about 50percent lead and 50 percent tin,

but containing 0.8 gram of zinc, were placed in a laboratory melting pot and heated to about 550 C. Then 3.4 grams of lead chloride were added .to the metal and stirred for several minutes. The metal was cooled and poured into a test mold, and the appearance of the bar showed that zinc was still present. The metal was again reheated to 550C. and about three grams of lead chloride again added to the molten metal, which was cooled ,to about 400 C., and poured out into bars. The solder metal now had the appearance of being practically free from zinc, and

it was found that the. zinc had been removed down to less than 0.002 percent. Since the original metal contained about 0.11 ,percent zinc, the

emciency of this two-step removal process was better than 98 percent. When the lead chloride reagent in theabove examples is mixed with a combination of melting point depressing salts in the manner of the present invention, the temperatures ofreaction will be relatively greatly reduced.

The foregoing description of my present invention has been given for clearness of understanding, and no undue limitations should be deduced therefrom, but the appended claims view of the prior art.

What I claim is:

1. A method of removing-impurities from tinny lead or lead-tin alloys which comprises melting such tinny lead or lead-tin alloys containing as impurities at least one element selected from the group consisting of the alkali or alkaline earth metals, magnesium, beryllium, aluminum and should be construed as broadly as po'ssibleln zinc, admixing therewith a lead halide mixed with a lesser amount of a mixture of sodium slag and separating the purified metal from the said halide slag.

2. A method of removing impurities from tinny lead or lead-tin alloyswhich comprises melting such tinny lead or lead-tin alloys containing as impurities at least one element selected from the group consisting of the alkali or alkaline earth metals, beryllium, aluminum and zinc, admixing therewith a lead halide mixed with a lesser amount of a mixture of at least I added 15 grams of lead chloride while stirring and continued to stir the molten mass for about one minute. I then drained of! a suilicient two alkali metal halides and agitating the mass at a temperature above the melting point of the melting such tinny lead or lead-tin alloys contaminated with aluminum, admixing therewith an amount of lead chloride calculated upon the amount of impurity to be removed from the metal, and admixed with a lesser amount of a mixture of at least two alkali metal halides, and agitating the mass at a temperature above the melting point of the lead chloride-alkali metal halides mixture until the aluminum is removed from the molten mass as an aluminum chloride slag and separating the purified metal fromthe said aluminum chloride slag.

4. A method of removing zinc from tinny leads and lead-tin alloys which comprises melting such tinny lead or lead-tin alloy contaminated with zinc, admixing therewith lead chloride mixed with a lesser amount of a mixture of at least two alkali metal halides and agitating the mass at a temperature above the melting point of the lead chloride-alkali metal halides GUSTAVE E. 1mm. 

